High frequency transformer (HFT) is the core magnetic component of power electronic transformer. Its insulation performance has a serious impact on the stability and security of power system. The transformer is subjected to high frequency square wave voltage with steep edges and high operating temperature. This exposure significantly influences the discharge behavior of the insulating material. To elucidate the effects of voltage frequency on breakdown characteristics, a square-wave loading platform was established. Epoxy resin and DMD paper, two widely utilized insulating materials, were selected for analysis. The investigation spanned frequencies ranging from 2 kHz to 14 kHz and temperatures between 20 ℃ to 120 ℃. The probability of insulation failure was assessed using the two-parameter Weibull statistical method. The correlation between breakdown voltage and frequency was examined. The voltage-frequency (V-f) and voltage-temperature (V-T) characteristics were analyzed. Experimental findings indicate that the breakdown voltage diminishes with an escalation in voltage frequency and with a rise in temperature. The voltage-temperature characteristic accords with the polymer free volume theory.